Ruthenium Complexes of 2,2′-Bipyridine-6,6′-diphosphonate Ligands for Water Oxidation

Two novel ruthenium complexes, [Ru(2,2'-bipyridine-6,6'-diphosphonato)(pic)(2)] (2) (pic = 4-picoline) and [Ru(6,6'-diisopropyl-2,2'-bipyridine-6,6'-diphosphonato)(pic)(2)] (3) bearing phosphonate groups have been synthesized and characterized by NMR spectroscopy, elemental analyses, X-ray crystallography, cyclic voltammetry, and UV/Vis spectroscopy. Both complexes show catalytic water oxidation activity by electrochemistry. At pH 7, the Ru-II/III redox couple of 2 is observed at a lower potential than that of 3, yet significantly, 3 oxidizes water at a lower onset potential. At pH1 however, 2 and 3 have comparable catalytic reactivity using sacrificial oxidant Ce-IV. We propose that water oxidation activities of 2 and 3 are influenced by overall charges. For example, in oxidizing from Ru-II to Ru-III at pH 7, 2 acquires a -1 overall charge whereas 3 acquires a +1 charge. Charge-dictated electrostatic effects may govern binding of a water molecule to the metal site.